It is common knowledge that a measure of perfect aerodynamic performance characteristics of an aircraft depends on the aerodynamic fineness (lift-to-drag) ratio K=C.sub.L /C.sub.D, where C.sub.L and C.sub.D are the aerodynamic lift and the aerodynamic drag coefficients, respectively.
It ensues from the definition of the aerodynamic fineness ratio that in order to attain high values of K it is necessary either to reduce the aerodynamic drag of an aircraft or to increase its aerodynamic lift.
The lift can be increased by increasing the angle of attack of the aircraft lifting surfaces. However, with an increased angle of attack, a positive pressure gradient arises on the trailing surface of the wings directed along the airflow about the wing. At definite levels of said positive pressure gradient the airflow is incapable of moving against the positive pressure gradient due to its having inadequate kinetic energy near the wing surface and is separated from the latter.
Such an airflow separation results in a badly increased aerodynamic drag of the aircraft construction members streamed with the airflow and in a reduced lift of the aerodynamic lifting surfaces (i.e., wings and fuselages), that is C.sub.D increases, C.sub.L decreases, with the result that the aerodynamic fineness ratio K is reduced too.
To ensure against aerodynamic stall and improve aircraft aerodynamic characteristics, as well as those of other aircraft, the wall boundary layers of the airflow are sucked off, thus increasing the kinetic energy of the wall boundary layer and its ability to overcome high pressure gradients.
The present state of the art includes a variety of practical solutions of the problem to the boundary layer control by sucking air off in the wall zone.
One of the prior-art methods for control of the boundary layer is known to effect air sucking off the wall zone through air-bleed orifices provided on the aerodynamic surface of an aircraft (cf. German Patent No. 1,273,338). The method is a highly energy-consuming one since air is bled from the wall zone in a direction normal to the boundary layer. The same disadvantage is inherent in a technical solution pertinent to an aircraft having its fuselage shaped as a thick short-span wing (cf. U.S. Pat. No. 3,077,321) equipped with a boundary layer control device which appears as a boundary-layer-control manifold situated in the aft fuselage and communicating, through suction slots, with the wall airflow zone. Provision is made in the inlet portion of the manifold for a rarefying arrangement to establish rarefaction in the manifold with the aid of a bank of suction fans. The system is, however, too power-consuming due to high power input of the fan drives required for air suction off the low-pressure zones on the aircraft surface and blowing the air in the high-pressure zones near the trailing edge of the aircraft.
Moreover, the required fan input power is increased due to an excessively large amount of air sucked off the low-pressure zone. According to the laws governing boundary layer control, the amount of sucked-off or blow-in air required for establishing a nonseparated airflow increases intensely downstream of the airflow towards the trailing edge. In the boundary layer control discussed above the amount of sucked-off air is equal to the amount of the air blown in the vicinity of the trailing edge. According to the aforesaid law, the amount of sucked-off air in the control system under consideration is to be several times lower than that of the air blown-in near the aircraft rear. Any violation of the boundary layer control law results in higher power consumption for fan drive and affects adversely the aircraft aerodynamic efficiency. An excessively high suction results in a rise of the skin-friction drag.
More advanced are a method and devices for control of the boundary layer, wherein the wall air layer is sucked off using special chambers established in the trailing aerodynamic surface, vortex flows are created in the interior of said chambers, the direction of which in the wall portion of the chamber coincides with the direction of the boundary layer whereby the velocity of the latter increases resulting in a nonseparated flow of an airfoil.
Known in the present state of the art is a device for the boundary layer control operating according to the method described before and having a number of vortex chambers located on the inner side of the air-foil and provided with holes arranged across the external airflow (cf. U.S. Pat. No. 4,671,474).
Vortex motion inside the chambers in maintained due to hydrodynamic interaction of the vortex motion inside the chamber with the external airflow in the zone of the suction holes and at the expense of the power of the air suction source.
However, said device suffers from said disadvantages the principal of which are sophisticated construction, high airfoil drag level, and highly power-consuming suction of the vortex flow.
High drag level results from a considerable airfoil drag due to the poorly streamlined square shape of the chamber and on account of an increased skin-friction drag on the surface of the vortex chambers.
Considerable power consumption for airflow suction is due to a large resistance offered by communication lines connecting the vertex chambers to a low-pressure source. The throttling effect of the communication lines is especially high with respect to a sonic flow made use of in the known device. In addition, at low velocities of the external airflow and small values of the positive pressure gradient, the power system of the device operates in an uneconomic mode, this being due to the fact that the system is adjusted for the maximum airflow velocity and pressure gradient values and therefore sucks air in excess of the necessary amount, which leads to unjustified power consumption.
One more prior-art device for boundary layer control is known to have cylinder-shaped vortex chambers, whereby their profile (form) drag can be reduced (cf. British Patent No. 2,178,131). However, it is due to a small size of the suction slot of communicating the airflow well boundary layer with the vortex chamber that the area of interaction of the airflow in the vortex chamber with the external airflow has but inadequate extent to provide a necessary increase of the airflow velocity in the wall boundary layer thereof to prevent boundary layer separation in case of great positive pressure gradients.